Better art or a better plot?

Recently I have run into quite a dilemma regarding the art-style of a manga versus the plot-line of a manga. One of my fellow anime lovers refused to read One Piece due to the “rudimentary eyes” of the characters and their grossly thick necks. Though her comments elicited a highly judgmental response from me (now I deign to recognize them as a true manga lover), I also became curious as to how important the art of a manga is.

Must the drawings in a manga be pristine in order for the reader to read the story, or can the plot-line make up for the lack of professional art? What are your thoughts?